CA2121969C - Pre-treatment of sulfite pulp before bleaching with oxygen containing chemicals - Google Patents
Pre-treatment of sulfite pulp before bleaching with oxygen containing chemicalsInfo
- Publication number
- CA2121969C CA2121969C CA002121969A CA2121969A CA2121969C CA 2121969 C CA2121969 C CA 2121969C CA 002121969 A CA002121969 A CA 002121969A CA 2121969 A CA2121969 A CA 2121969A CA 2121969 C CA2121969 C CA 2121969C
- Authority
- CA
- Canada
- Prior art keywords
- pulp
- oxygen
- bleached
- caustic extraction
- oxygen containing
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
Links
Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B27—WORKING OR PRESERVING WOOD OR SIMILAR MATERIAL; NAILING OR STAPLING MACHINES IN GENERAL
- B27K—PROCESSES, APPARATUS OR SELECTION OF SUBSTANCES FOR IMPREGNATING, STAINING, DYEING, BLEACHING OF WOOD OR SIMILAR MATERIALS, OR TREATING OF WOOD OR SIMILAR MATERIALS WITH PERMEANT LIQUIDS, NOT OTHERWISE PROVIDED FOR; CHEMICAL OR PHYSICAL TREATMENT OF CORK, CANE, REED, STRAW OR SIMILAR MATERIALS
- B27K1/00—Damping wood
-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
- D21C1/00—Pretreatment of the finely-divided materials before digesting
-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
- D21C1/00—Pretreatment of the finely-divided materials before digesting
- D21C1/06—Pretreatment of the finely-divided materials before digesting with alkaline reacting compounds
-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
- D21C1/00—Pretreatment of the finely-divided materials before digesting
- D21C1/08—Pretreatment of the finely-divided materials before digesting with oxygen-generating compounds
-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
- D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
- D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
- D21C9/10—Bleaching ; Apparatus therefor
- D21C9/147—Bleaching ; Apparatus therefor with oxygen or its allotropic modifications
-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
- D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
- D21C9/10—Bleaching ; Apparatus therefor
- D21C9/16—Bleaching ; Apparatus therefor with per compounds
Landscapes
- Life Sciences & Earth Sciences (AREA)
- Engineering & Computer Science (AREA)
- Wood Science & Technology (AREA)
- Forests & Forestry (AREA)
- Paper (AREA)
Abstract
This invention relates to the pre-treatment of sulfite pulp. More particularly, this invention relates to a novel process for caustic extraction of unbleached sulfite pulp to reduce metallic impurities in the pulp.
The process of reducing metallic impurities in unbleached sulfite pulp comprises subjecting the pulp to caustic extraction.
The process of reducing metallic impurities in unbleached sulfite pulp comprises subjecting the pulp to caustic extraction.
Description
PRE-TREATMENT OF SULFITE PULP BEFORE
RT-T~'~G~TNG WITH OkY~--. CONTAINING CHEMICALS
FIELD OF THE INVENTION
This invention relates to the pre-treatment of sulfite pulp. More particularly, this invention relates to a novel process for caustic extraction of unbleached sulfite pulp to reduce metallic impurities in the pulp.
BACKGROUND OF THE INVENTION
Although wood is primarily composed of organic materials, it always contains some inorganic metallic impurities, which adversely affect the bleachability and bleach chemical consumption of the chemical pulp produced from wood. This is particularly important where hydrogen peroxide, oxygen or ozone is used as the bleaching chemi-cal. Therefore, for effective and economic bleaching of chemical pulp, it is sometimes necessary to remove the metallic impurities from the pulp. This is normally achieved by chelation and chelating compounds such as EDTA
or DTPA, which tie up with the metals and thereby remove them from the pulp.
Even with the assistance of chelating compounds, the removal of metallic impurities from pulp is sometimes difficult. In fact, the pulp has to be treated with the right amount of a chelating compound at an optimum pH and temperature, and then thoroughly washed with water in order to remove the metals effectively. An effective chelation and peroxide stage combination to produce high brightness pulp has been described in the literature as the Lignox process.
A number of patents have been granted over the years disclosing various proposals for treating unbleached sulfite pulp. C~n~;an Patent No. 996,312, issued September 7, 1976, ITT Industries, Inc. discloses a process 9~q _ - 2 for producing refined and bleached wood pulp which com-prises digesting wood in a sodium-base acid sulfite diges-tion liquor to produce a completely digested, unwashed, unbleached sodium-base acid sulfite pulp. The unwashed, unbleached acid sulfite pulp is separated from the result-ing spent sulfite digestion liquor. Without subjecting the pulp to a water wash, the unwashed, unbleached acid sulfite pulp is treated in a hot caustic extraction refining step with sodium hydroxide in an amount of about 3-35~ (based on the weight of oven dried pulp) at a temperature of about 40-150C for a period of about 15-90 minutes. This pro-duces an unwashed, unbleached, chemically refined pulp.
The chemically refined pulp is separated from the hot caustic extraction effluent resulting from said wash. The refined pulp is then bleached. This patent does not dis-close washed, unbleached pulp. The process focuses on the recovery of caustic soda for a caustic soda basis process.
The process, being a caustic soda based process, does not contemplate or relate to ammonia based processes. An ammonia based process and a caustic soda based process are re~l~n~nt relative to one another. This patent does not disclose that caustic extraction can unexpectedly reduce metal content in the washed pulp.
SUMMARY OF THE INVENTION
The invention is directed to a process of reduc-ing metallic impurities in unbleached sulfite pulp which comprises subjecting the pulp to caustic extraction. In the process, the caustic extraction can be conducted under pressurized extraction at a temperature between about 105C
to 145C and at above atmospheric pressure. The caustic extraction can be conducted with caustic soda between 4 to 10~ wt. concentration. The caustic extraction of the unbleached sulfite pulp can be followed by an oxygen containing bleaching stage. The oxygen containing bleach-ing stage can be conducted with hydrogen peroxide, oxygen ~.
2121~69 _ - 3 -or ozone, individually, or in any combination. The pulp can be washed unbleached pulp.
In the process, the caustic extraction of the unbleached sulfite pulp can also be followed by an acidic wash stage. The acid washed pulp can then be bleached by an oxygen containing bleaching agent. The oxygen contain-ing bleaching agent can be hydrogen peroxide, oxygen or ozone, individually, or in any combination.
In the process, the caustic extraction of the unbleached sulfite pulp can be followed by a chelation stage. The resulting pulp can then be bleached by an oxygen containing bleaching agent like hydrogen peroxide, oxygen or ozone, individually, or in any combination.
DRAWINGS
In drawings which illustrate specific embodiments of the invention, but which should not be construed as restricting the spirit or scope of the invention in any way:
Figure la illustrates a schemtaic flow sheet of a first alternative method of practicing the process of the invention.
Figure lb illustrates a schematic flow sheet of a second alternative method of practicing the process of the invention.
Figure lc illustrates a schematic flow sheet of a third alternative method of practicing the process of the invention.
_ -- 4 DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
OF THE INVENTION
Although wood is primarily composed of organic materials, it always contains some inorganic metallic impurities coming primarily from the soil where the tree has grown. When wood is chemically pulped, it still retains some of these metals in the unbleached pulp. Other metals may also come from the cooking chemicals, water or from corrosion of storage tanks or equipment. The pres-ence of these metallic impurities, such as manganese, iron and copper, particularly manganese, disadvantageously affect the bleachability and bleach consumption of the pulp, where oxygen containing bleaching chemicals such as oxygen, ozone or hydrogen peroxide are used.
It has been argued that acid sulfite pulp should not have metallic impurities, because the acid used for pulping removes these metals during the cooking process.
However, unbleached acid sulfite pulp may contain suffi-cient amounts of metals to substantially affect its bleach-ing by oxygen containing bleaching agents. We have found that the manganese content of acid sulfite pulp of pulp mills located on the westcoast region of Canada can vary between 20 to 40 ppm, where hemlock is used as the raw material.
Both cellulose and lignin have been known to be very good absorbents of heavy metals. Although metals may be dissolved out in the cooking liquor during the pulping process, the liquor associated with the blown pulp, due to recycling of process water in the washing and screening plant, can enrich the metal concentration and redeposit these metals on the fibres. We have found that the ray cells removed by side hill screens in the screening plant, with their large surface to volume ratios, contain more metals than the bulk of the pulp. This again shows the q (D q _- - 5 importance of the absorption phenomenon in binding the metals. From a number of measurements carried out over several days, we have noted an average manganese content of 49.3 ppm in the side hill screen rejects as compared with 26.5 ppm manganese in the washed and screened pulp.
It is necessary to remove these metallic impur-ities from the unbleached pulp, whether made by kraft or sulfite process, to effectively and economically bleach the pulp by oxygen containing bleaching agents. Conventional-ly, this can be achieved by acid washing which can desorb the metals from the pulp, or by application of chelating agents which tie up the metals and thereby remove them from the pulp. The pulp must be treated with the right amount of a chelating compound at an optimum pH and temperature and then be thoroughly washed with water in order to remove the metals effectively.
A large number of studies have been conducted on the pretreatment of kraft pulps and the use of acidic wash and chelates like EDTA or DTPA to reduce the metal contami-nation of the unbleached pulp. It has been found that either a low pH acid wash or a pH-optimized chelate treat-ment will generally remove the metals adequately from a kraft pulp. However, the manganese content could not be completely removed by such methods. In one such example (Tappi, 76, No. 8, page 194) superchelation by three successive treatments of 10~ wt. DTPA only reduced the manganese content of the kraft pulp to 3 ppm.
We have developed a pulp treating sequence starting with a hot caustic extraction of washed unbleached sulfite pulp, which is obtained from an ammonia-base acid sulfite cooking process, and then bleaching the resultant pulp with oxygen or alkali oxygen bleaching as the next stage. We have discovered surprisingly that hot caustic extraction provides good removal of metallic impurities q~q _~ - 6 from the unbleached pulp. This enhances the efficiency of the following bleaching sequence and reduces the costs of a chelation stage, if one is to be used.
Figure la illustrates a first way of practicing the invention. Washed, unbleached pulp is subjected to caustic extraction to yield extracted pulp. The extracted pulp is subjected to oxygen bleaching to yield bleached pulp. The bleaching agent is an oxygen bleaching agent such as hydrogen peroxide, oxygen or ozone, individually, or in any combination.
Figure lb illustrates a second alternative method of practicing the invention. Washed, unbleached pulp is subjected to caustic extraction to yield extracted pulp.
The extracted pulp is subjected to chelate treatment to yield treated pulp which is then bleached to yield bleached pulp.
Figure lc illustrates a third alternative method of practicing the invention. Washed, unbleached pulp is subjected to caustic extraction. The extracted pulp is then given an acid treatment. The acid treated pulp is then bleached to yield bleached pulp.
Unbleached pulp was obtained from the Port Alice mill of Western Pulp Inc. Unlike kraft pulp, acid wash by itself is not as effective in lowering metal content as the chelate treatment. In the case of the pulp we tested, a sulfuric acid wash removed 31% of manganese at pH 2.5, 60%
at pH 2 and 70% at pH 1.7. Even with 70% removal, the manganese content was still 9.4 ppm. Therefore, acidic wash as such is not satisfactory for sulfite pulp for further bleaching by oxygen containing bleaching agents.
However, with sulfite pulp, we have found that a 0.2% wt.
application of the chelate DTPA at a neutral (pH 7) can reduce the manganese content of pulp to 1 ppm or less. The results of these tests are tabulated in Table 1 below.
EFFECT OF CHELATE TREATMENT AND ACID WASH
Ot~ UNBLEACHED SU~FITE PUU AT DIFFERENT pH
~pl~Y -~ r~aooen~ ~$~
16.1 DTPA 02 2.0 9.8 .5 DTPA 0~ 7.0 0.25 33.5 DTPA 0.04 7.0 112 33.5 H2SO4 1.1 2.5 232 33.5 H2SO4 42 1 20 13.5 33.5 H2SO4 10.8 1-7 94 Specifically, in the course of our studies, we have unexpectedly discovered that a caustic extraction of the washed sulfite pulp can reduce the metal content substantially. When hot caustic extraction was carried out with 6% wt. caustic soda at 122C and a corresponding elevated pressure for 40 minutes, we found that the manga-nese content dropped to less than half of the originalvalue while the K.No. dropped by about 25%. Pulp metal levels dropped with its lignin content.
To prove that lignin is an important factor in binding of metals to pulp, we carried out complete deligni-fication of the pulp by acid chlorite treatment. We found that with complete delignification, the pulp K.No. was less than 0.5 and the manganese content was reduced from 38 ppm to 1 ppm or less. These results, compared with controls, are shown in Table 2 below.
REMOVAL OF MANGANESE FROM UNBLEACHED SULFITE PULP BY 100% DELIGNIFICATION
Two Sta~e Delignification by~e Chlorite l lo'oce"u'~e Method Star~ng Mat~rial - Unbleached Sulfite Pulp wffll 33 5 ppm Manganes~
Chlorite 1 2 3 4 Treatments100% Delig~') Control~100~C Delig(') Control~
Time, min 90+60 90+60 90+60 90+60 Tel"pe,~ture, C 70 70 70 70 Pulp, o.d.g. 20 20 50 50 NaClO2g 100+9 0+0 250+22.5 0+0 Glacialaceticacid,mL 100+6 100+6 250+15 250+15 Dis~lled water, mL800+800 800+800 2000+2000 2000+2000 Fmal pH - Stage 1 3.4 1.7 4.0 2.5 Fmal pH - Stage 2 3.1 2.6 3.4 2.9 K.No. (% reduction)0.5 (9n18.4 (2) 0.3 (98) 17.7 (6) Mn, ppm (~6 removal) 1.0 (9n 26.0 (22) 0-9 (9n 24.0 (28) 1: Chlorite tréatrnent was done in ~o sta~es wWl washin~ bet~en sta~es 2: Two - stage control w;~out any sodium chlorite added We have also discovered that after a caustic extraction stage, it is easier to remove the metals from the resulting pulp by both chelate treatment and acid wash.
Chelating compounds are expensive and this invention provides a way of saving the cost of chelants. An acidic wash at a pH of 2 reduced the man~anese to 1.7 ppm, so a simple acidic wash instead of a chelate treatment may be sufficient in many cases. Table 3, which is a composite of three sequences of tests, tabulates the data obtained from conducting these tests, compared to a blank (control).
al~l9~q DTPA DOSAGE OPTIMIZATION FOR INITIAL AND POST E-STAGE CHELATION
r- Vc-~-, -U~chec ~ W~5 ~- V,~
B 2~ F~ ~ C ~ e (i)(ii) Dosage('), % -- 0.20.04 2.8 Temperature, C 20 20 20 30 10 Initial pH . 5.5 . 2.0 - Mn, ppm 33.40.2511.2 15.4 Alkaline Purification S-age: 12.5% Consi~,tency, 12'`C, 40 mil, 6.0% NaOH, 0.1% Antarox Final pH -- -- -- -- 9 9 15 25mLKno. -- -- -- -- 12.6 Intrinsic Viscosity - ---- -- 13.3 - Mn, ppm -- -- -- -- 15.9 T or Acid Sta~e: 2% Consistenc~, 30 min (i)(ii) (iii) (iv) DTPA, % -- -- -- -- 0.040.04 0.02 HCI, % -- -- -- -- -- -- -- 2.6 Temperaturel C -- -- -- - 20 20 20 30 Inital pH -- -- -- -- 7.0 8.0 7.0 2.0 - Mn, ppm -- -- -- --0.81 1.2 9.2 1.6 25 Mn removal, % 0 99 67 54 98 96 73 95 The first sequence of four tests shown in Table 3 shows that the chelate DTPA per-forms best at the neutral pH of 7 and that a 0.2~ wt. addition of DTPA to the pulp reduces the Mn content to less than 1 ppm. The second sequence in Table 3 shows that the Mn content is reduced to less than half compared to the blank by caustic extraction under the conditions shown in Table 3. The third four test sequence in Table 3 shows that after caustic extraction the Mn content can be reduced to 1.6 ppm by 2.6~ wt. HCl and as low as 0.81 ppm by 0.04~ wt. DTPA.
~ l~l9 ~9 In certain situations, it is possible that a caustic extracted pulp, even without further chelate treatment or acid wash, can be bleached fairly satisfactor-ily by oxygen containing bleaching chemicals in spite of the presence of some minor amounts of residual metals.
As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.
RT-T~'~G~TNG WITH OkY~--. CONTAINING CHEMICALS
FIELD OF THE INVENTION
This invention relates to the pre-treatment of sulfite pulp. More particularly, this invention relates to a novel process for caustic extraction of unbleached sulfite pulp to reduce metallic impurities in the pulp.
BACKGROUND OF THE INVENTION
Although wood is primarily composed of organic materials, it always contains some inorganic metallic impurities, which adversely affect the bleachability and bleach chemical consumption of the chemical pulp produced from wood. This is particularly important where hydrogen peroxide, oxygen or ozone is used as the bleaching chemi-cal. Therefore, for effective and economic bleaching of chemical pulp, it is sometimes necessary to remove the metallic impurities from the pulp. This is normally achieved by chelation and chelating compounds such as EDTA
or DTPA, which tie up with the metals and thereby remove them from the pulp.
Even with the assistance of chelating compounds, the removal of metallic impurities from pulp is sometimes difficult. In fact, the pulp has to be treated with the right amount of a chelating compound at an optimum pH and temperature, and then thoroughly washed with water in order to remove the metals effectively. An effective chelation and peroxide stage combination to produce high brightness pulp has been described in the literature as the Lignox process.
A number of patents have been granted over the years disclosing various proposals for treating unbleached sulfite pulp. C~n~;an Patent No. 996,312, issued September 7, 1976, ITT Industries, Inc. discloses a process 9~q _ - 2 for producing refined and bleached wood pulp which com-prises digesting wood in a sodium-base acid sulfite diges-tion liquor to produce a completely digested, unwashed, unbleached sodium-base acid sulfite pulp. The unwashed, unbleached acid sulfite pulp is separated from the result-ing spent sulfite digestion liquor. Without subjecting the pulp to a water wash, the unwashed, unbleached acid sulfite pulp is treated in a hot caustic extraction refining step with sodium hydroxide in an amount of about 3-35~ (based on the weight of oven dried pulp) at a temperature of about 40-150C for a period of about 15-90 minutes. This pro-duces an unwashed, unbleached, chemically refined pulp.
The chemically refined pulp is separated from the hot caustic extraction effluent resulting from said wash. The refined pulp is then bleached. This patent does not dis-close washed, unbleached pulp. The process focuses on the recovery of caustic soda for a caustic soda basis process.
The process, being a caustic soda based process, does not contemplate or relate to ammonia based processes. An ammonia based process and a caustic soda based process are re~l~n~nt relative to one another. This patent does not disclose that caustic extraction can unexpectedly reduce metal content in the washed pulp.
SUMMARY OF THE INVENTION
The invention is directed to a process of reduc-ing metallic impurities in unbleached sulfite pulp which comprises subjecting the pulp to caustic extraction. In the process, the caustic extraction can be conducted under pressurized extraction at a temperature between about 105C
to 145C and at above atmospheric pressure. The caustic extraction can be conducted with caustic soda between 4 to 10~ wt. concentration. The caustic extraction of the unbleached sulfite pulp can be followed by an oxygen containing bleaching stage. The oxygen containing bleach-ing stage can be conducted with hydrogen peroxide, oxygen ~.
2121~69 _ - 3 -or ozone, individually, or in any combination. The pulp can be washed unbleached pulp.
In the process, the caustic extraction of the unbleached sulfite pulp can also be followed by an acidic wash stage. The acid washed pulp can then be bleached by an oxygen containing bleaching agent. The oxygen contain-ing bleaching agent can be hydrogen peroxide, oxygen or ozone, individually, or in any combination.
In the process, the caustic extraction of the unbleached sulfite pulp can be followed by a chelation stage. The resulting pulp can then be bleached by an oxygen containing bleaching agent like hydrogen peroxide, oxygen or ozone, individually, or in any combination.
DRAWINGS
In drawings which illustrate specific embodiments of the invention, but which should not be construed as restricting the spirit or scope of the invention in any way:
Figure la illustrates a schemtaic flow sheet of a first alternative method of practicing the process of the invention.
Figure lb illustrates a schematic flow sheet of a second alternative method of practicing the process of the invention.
Figure lc illustrates a schematic flow sheet of a third alternative method of practicing the process of the invention.
_ -- 4 DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS
OF THE INVENTION
Although wood is primarily composed of organic materials, it always contains some inorganic metallic impurities coming primarily from the soil where the tree has grown. When wood is chemically pulped, it still retains some of these metals in the unbleached pulp. Other metals may also come from the cooking chemicals, water or from corrosion of storage tanks or equipment. The pres-ence of these metallic impurities, such as manganese, iron and copper, particularly manganese, disadvantageously affect the bleachability and bleach consumption of the pulp, where oxygen containing bleaching chemicals such as oxygen, ozone or hydrogen peroxide are used.
It has been argued that acid sulfite pulp should not have metallic impurities, because the acid used for pulping removes these metals during the cooking process.
However, unbleached acid sulfite pulp may contain suffi-cient amounts of metals to substantially affect its bleach-ing by oxygen containing bleaching agents. We have found that the manganese content of acid sulfite pulp of pulp mills located on the westcoast region of Canada can vary between 20 to 40 ppm, where hemlock is used as the raw material.
Both cellulose and lignin have been known to be very good absorbents of heavy metals. Although metals may be dissolved out in the cooking liquor during the pulping process, the liquor associated with the blown pulp, due to recycling of process water in the washing and screening plant, can enrich the metal concentration and redeposit these metals on the fibres. We have found that the ray cells removed by side hill screens in the screening plant, with their large surface to volume ratios, contain more metals than the bulk of the pulp. This again shows the q (D q _- - 5 importance of the absorption phenomenon in binding the metals. From a number of measurements carried out over several days, we have noted an average manganese content of 49.3 ppm in the side hill screen rejects as compared with 26.5 ppm manganese in the washed and screened pulp.
It is necessary to remove these metallic impur-ities from the unbleached pulp, whether made by kraft or sulfite process, to effectively and economically bleach the pulp by oxygen containing bleaching agents. Conventional-ly, this can be achieved by acid washing which can desorb the metals from the pulp, or by application of chelating agents which tie up the metals and thereby remove them from the pulp. The pulp must be treated with the right amount of a chelating compound at an optimum pH and temperature and then be thoroughly washed with water in order to remove the metals effectively.
A large number of studies have been conducted on the pretreatment of kraft pulps and the use of acidic wash and chelates like EDTA or DTPA to reduce the metal contami-nation of the unbleached pulp. It has been found that either a low pH acid wash or a pH-optimized chelate treat-ment will generally remove the metals adequately from a kraft pulp. However, the manganese content could not be completely removed by such methods. In one such example (Tappi, 76, No. 8, page 194) superchelation by three successive treatments of 10~ wt. DTPA only reduced the manganese content of the kraft pulp to 3 ppm.
We have developed a pulp treating sequence starting with a hot caustic extraction of washed unbleached sulfite pulp, which is obtained from an ammonia-base acid sulfite cooking process, and then bleaching the resultant pulp with oxygen or alkali oxygen bleaching as the next stage. We have discovered surprisingly that hot caustic extraction provides good removal of metallic impurities q~q _~ - 6 from the unbleached pulp. This enhances the efficiency of the following bleaching sequence and reduces the costs of a chelation stage, if one is to be used.
Figure la illustrates a first way of practicing the invention. Washed, unbleached pulp is subjected to caustic extraction to yield extracted pulp. The extracted pulp is subjected to oxygen bleaching to yield bleached pulp. The bleaching agent is an oxygen bleaching agent such as hydrogen peroxide, oxygen or ozone, individually, or in any combination.
Figure lb illustrates a second alternative method of practicing the invention. Washed, unbleached pulp is subjected to caustic extraction to yield extracted pulp.
The extracted pulp is subjected to chelate treatment to yield treated pulp which is then bleached to yield bleached pulp.
Figure lc illustrates a third alternative method of practicing the invention. Washed, unbleached pulp is subjected to caustic extraction. The extracted pulp is then given an acid treatment. The acid treated pulp is then bleached to yield bleached pulp.
Unbleached pulp was obtained from the Port Alice mill of Western Pulp Inc. Unlike kraft pulp, acid wash by itself is not as effective in lowering metal content as the chelate treatment. In the case of the pulp we tested, a sulfuric acid wash removed 31% of manganese at pH 2.5, 60%
at pH 2 and 70% at pH 1.7. Even with 70% removal, the manganese content was still 9.4 ppm. Therefore, acidic wash as such is not satisfactory for sulfite pulp for further bleaching by oxygen containing bleaching agents.
However, with sulfite pulp, we have found that a 0.2% wt.
application of the chelate DTPA at a neutral (pH 7) can reduce the manganese content of pulp to 1 ppm or less. The results of these tests are tabulated in Table 1 below.
EFFECT OF CHELATE TREATMENT AND ACID WASH
Ot~ UNBLEACHED SU~FITE PUU AT DIFFERENT pH
~pl~Y -~ r~aooen~ ~$~
16.1 DTPA 02 2.0 9.8 .5 DTPA 0~ 7.0 0.25 33.5 DTPA 0.04 7.0 112 33.5 H2SO4 1.1 2.5 232 33.5 H2SO4 42 1 20 13.5 33.5 H2SO4 10.8 1-7 94 Specifically, in the course of our studies, we have unexpectedly discovered that a caustic extraction of the washed sulfite pulp can reduce the metal content substantially. When hot caustic extraction was carried out with 6% wt. caustic soda at 122C and a corresponding elevated pressure for 40 minutes, we found that the manga-nese content dropped to less than half of the originalvalue while the K.No. dropped by about 25%. Pulp metal levels dropped with its lignin content.
To prove that lignin is an important factor in binding of metals to pulp, we carried out complete deligni-fication of the pulp by acid chlorite treatment. We found that with complete delignification, the pulp K.No. was less than 0.5 and the manganese content was reduced from 38 ppm to 1 ppm or less. These results, compared with controls, are shown in Table 2 below.
REMOVAL OF MANGANESE FROM UNBLEACHED SULFITE PULP BY 100% DELIGNIFICATION
Two Sta~e Delignification by~e Chlorite l lo'oce"u'~e Method Star~ng Mat~rial - Unbleached Sulfite Pulp wffll 33 5 ppm Manganes~
Chlorite 1 2 3 4 Treatments100% Delig~') Control~100~C Delig(') Control~
Time, min 90+60 90+60 90+60 90+60 Tel"pe,~ture, C 70 70 70 70 Pulp, o.d.g. 20 20 50 50 NaClO2g 100+9 0+0 250+22.5 0+0 Glacialaceticacid,mL 100+6 100+6 250+15 250+15 Dis~lled water, mL800+800 800+800 2000+2000 2000+2000 Fmal pH - Stage 1 3.4 1.7 4.0 2.5 Fmal pH - Stage 2 3.1 2.6 3.4 2.9 K.No. (% reduction)0.5 (9n18.4 (2) 0.3 (98) 17.7 (6) Mn, ppm (~6 removal) 1.0 (9n 26.0 (22) 0-9 (9n 24.0 (28) 1: Chlorite tréatrnent was done in ~o sta~es wWl washin~ bet~en sta~es 2: Two - stage control w;~out any sodium chlorite added We have also discovered that after a caustic extraction stage, it is easier to remove the metals from the resulting pulp by both chelate treatment and acid wash.
Chelating compounds are expensive and this invention provides a way of saving the cost of chelants. An acidic wash at a pH of 2 reduced the man~anese to 1.7 ppm, so a simple acidic wash instead of a chelate treatment may be sufficient in many cases. Table 3, which is a composite of three sequences of tests, tabulates the data obtained from conducting these tests, compared to a blank (control).
al~l9~q DTPA DOSAGE OPTIMIZATION FOR INITIAL AND POST E-STAGE CHELATION
r- Vc-~-, -U~chec ~ W~5 ~- V,~
B 2~ F~ ~ C ~ e (i)(ii) Dosage('), % -- 0.20.04 2.8 Temperature, C 20 20 20 30 10 Initial pH . 5.5 . 2.0 - Mn, ppm 33.40.2511.2 15.4 Alkaline Purification S-age: 12.5% Consi~,tency, 12'`C, 40 mil, 6.0% NaOH, 0.1% Antarox Final pH -- -- -- -- 9 9 15 25mLKno. -- -- -- -- 12.6 Intrinsic Viscosity - ---- -- 13.3 - Mn, ppm -- -- -- -- 15.9 T or Acid Sta~e: 2% Consistenc~, 30 min (i)(ii) (iii) (iv) DTPA, % -- -- -- -- 0.040.04 0.02 HCI, % -- -- -- -- -- -- -- 2.6 Temperaturel C -- -- -- - 20 20 20 30 Inital pH -- -- -- -- 7.0 8.0 7.0 2.0 - Mn, ppm -- -- -- --0.81 1.2 9.2 1.6 25 Mn removal, % 0 99 67 54 98 96 73 95 The first sequence of four tests shown in Table 3 shows that the chelate DTPA per-forms best at the neutral pH of 7 and that a 0.2~ wt. addition of DTPA to the pulp reduces the Mn content to less than 1 ppm. The second sequence in Table 3 shows that the Mn content is reduced to less than half compared to the blank by caustic extraction under the conditions shown in Table 3. The third four test sequence in Table 3 shows that after caustic extraction the Mn content can be reduced to 1.6 ppm by 2.6~ wt. HCl and as low as 0.81 ppm by 0.04~ wt. DTPA.
~ l~l9 ~9 In certain situations, it is possible that a caustic extracted pulp, even without further chelate treatment or acid wash, can be bleached fairly satisfactor-ily by oxygen containing bleaching chemicals in spite of the presence of some minor amounts of residual metals.
As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.
Claims (16)
1. A process of reducing metallic impur-ities in unbleached sulfite pulp which comprises subjecting the pulp to caustic extraction.
2. A process as claimed in claim 1 wherein the unbleached sulfite pulp is washed pulp.
3. A process as claimed in claim 1 wherein the caustic extraction is conducted under pressurized extraction at a temperature between about 105°C to 145°C, and above atmospheric pressure.
4. A process as claimed in claim 1 wherein the caustic extraction is conducted with caustic soda between 4 to 10 % wt. concentration.
5. A process as claimed in claim 3 wherein the caustic extraction is conducted with caustic soda between 4 to 10 % wt. concentration.
6. A process as claimed in claim 1 wherein the extracted pulp yielded by the caustic extraction of the unbleached sulfite pulp is bleached by an oxygen contain-ing bleach to yield a bleached pulp.
7. A process as claimed in claim 6 wherein the oxygen containing bleach is conducted with hydrogen peroxide, oxygen or ozone, individually, or in any combina-tion.
8. A process as claimed in claim 5 wherein the extracted pulp yielded by the caustic extraction of the unbleached sulfite pulp is bleached by an oxygen contain-ing bleach to yield a bleached pulp.
9. A process as claimed in claim 8 wherein the oxygen containing bleach is conducted with hydrogen peroxide, oxygen or ozone, individually, or in any combina-tion.
10. A process as claimed in claim 1 wherein the extracted pulp yielded by the caustic extraction of the unbleached sulfite pulp is followed by an acid wash to yield an acid treated pulp.
11. A process as claimed in claim 10 wherein the pulp from the acid wash is bleached by an oxygen containing bleaching agent.
12. A process as claimed in claim 11 wherein the oxygen containing bleaching agent is hydrogen peroxide, oxygen or ozone, individually, or in any combi-nation.
13. A process as claimed in claim 1 wherein the extracted pulp yielded by the caustic extraction of the unbleached sulfite pulp is treated with a chelating agent to yield a chelate treated pulp.
14. A process as claimed in claim 13 wherein the chelate treated pulp is bleached by an oxygen containing bleaching agent.
15. A process as claimed in claim 14 wherein the oxygen containing bleaching agent is hydrogen peroxide, oxygen or ozone, individually, or in any combi-nation.
16. A process as claimed in claim 5 wherein the caustic extraction of the unbleached sulfite pulp is followed by a chelate treatment, the resulting treated pulp then being bleached by an oxygen containing bleaching agent like hydrogen peroxide, oxygen or ozone, individually, or in any combination.
Priority Applications (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002121969A CA2121969C (en) | 1994-04-22 | 1994-04-22 | Pre-treatment of sulfite pulp before bleaching with oxygen containing chemicals |
| ZA952751A ZA952751B (en) | 1994-04-22 | 1995-04-04 | Pre-treatment of sulfite pulp before bleaching with oxygen containing chemicals |
| NO951464A NO951464L (en) | 1994-04-22 | 1995-04-19 | Process for the treatment of sulfite pulp for bleaching with oxygen-containing chemicals |
| JP7123116A JPH0841791A (en) | 1994-04-22 | 1995-04-24 | Method for pretreatment of sulfite pulp for bleaching with oxygen-containing chemical substance |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA002121969A CA2121969C (en) | 1994-04-22 | 1994-04-22 | Pre-treatment of sulfite pulp before bleaching with oxygen containing chemicals |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2121969A1 CA2121969A1 (en) | 1995-10-23 |
| CA2121969C true CA2121969C (en) | 1997-11-04 |
Family
ID=4153447
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002121969A Expired - Fee Related CA2121969C (en) | 1994-04-22 | 1994-04-22 | Pre-treatment of sulfite pulp before bleaching with oxygen containing chemicals |
Country Status (4)
| Country | Link |
|---|---|
| JP (1) | JPH0841791A (en) |
| CA (1) | CA2121969C (en) |
| NO (1) | NO951464L (en) |
| ZA (1) | ZA952751B (en) |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP4641629B2 (en) * | 2001-01-26 | 2011-03-02 | 日本製紙株式会社 | Cellulose fiber material pulp bleaching method |
-
1994
- 1994-04-22 CA CA002121969A patent/CA2121969C/en not_active Expired - Fee Related
-
1995
- 1995-04-04 ZA ZA952751A patent/ZA952751B/en unknown
- 1995-04-19 NO NO951464A patent/NO951464L/en unknown
- 1995-04-24 JP JP7123116A patent/JPH0841791A/en not_active Withdrawn
Also Published As
| Publication number | Publication date |
|---|---|
| NO951464L (en) | 1995-10-23 |
| JPH0841791A (en) | 1996-02-13 |
| ZA952751B (en) | 1995-12-21 |
| NO951464D0 (en) | 1995-04-19 |
| CA2121969A1 (en) | 1995-10-23 |
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| EEER | Examination request | ||
| MKLA | Lapsed |